This invention relates to a transistor bridge for driving a motor, including a short-circuit detector for shutting down the bridge driver upon detection of a fault, and more particularly to such a bridge that includes a circuit for clamping the bridge current by chopping the drive-signal to the bridge transistors, especially during starting, to a value less than that which triggers the short-circuit detector to shut off the bridge transistors.
Various load fault protection circuits have been used in motor drivers for sensing the driver current, or only the motor current, and shutting down the motor driver when the sensed current exceeds a predetermined value. Some electrical loads, and particularly motors, draw a large amount of current during the starting phase. The large starting current typically decays to a steady state operating value corresponding to a normal running condition.
To prevent a false indication of an electrical fault in a load having a high starting in-rush current, some electrical short circuit detectors have included time-delay circuits that disable the fault detector during the usual period of high in-rush current at starting, exposing the bridge to possible destruction for a short interval during starting.
Another solution to this problem is provided by a load driver with a short circuit protection circuit wherein during the motor-starting-current in-rush period, the threshold at which the short circuit detector triggers load shut off is itself modified, e.g. raised to temporarily allow starting with higher than normal-running motor current without triggering driver shut-off by the short circuit protection circuit.
To take into account tolerances in the motor starting current, this altered threshold must be raised well over the nominal start peak current, reducing the degree of protection. It also requires large time constants which are difficult to achieve in integrated circuit drivers.
It is an object of this invention to provide a full-wave-motor driver having a short circuit current detector that shuts down the bridge under all motor operating conditions.
It is a further object of this invention to provide such a motor driver having an operating mode that is independant of of tolerances in the peak motor starting current.
It is yet a further object of this invention to provide such a motor driver that includes circuits requiring no large time constants.